One task that arises during the use of cardiac rhythm management devices, including pacemakers, is that of sensing respiration. Determining the occurrence of respiration can be used to synchronously adapt pacing or defibrillation to the respiratory cycle.
The ability to operate in synchronism with a respiration signal can be used to advantage in many areas, including but not limited to respiratory sinus arrhythmia (RSA), defibrillation energy thresholds, and minute ventilation. For example, research indicates that pacing without RSA requires a higher number of beats to accomplish the same volume of oxygen delivery. Thus, proper synchronization can improve the efficiency of pulmonary gas exchange. Other research shows that synchronizing a defibrillation shock with expiration may decrease the defibrillation threshold. Finally, an improved respiration signal might provide a greater ventilation-to-cardiac component for more accurate baseline minute ventilation measurement in cases where the ventilation signal is significantly smaller than the cardiac component.
Conventional methods of sensing respiration involve measuring the impedance which arises between a ventricular lead tip electrode and an indifferent electrode on a pulse generator header. The signal obtained includes a constant component, a respiratory component, and a cardiac stroke component. The respiration component is then separated from the other components by filtering, which creates a time delay between the actual occurrence of respiration and provision of the extracted signal. The delay reduces the ability to fashion therapy according to the synchronous ideal. However, if respiratory signal quality can be improved, the need for filtering might be reduced or even eliminated, allowing cardiac therapy modification to occur in a more truly synchronous fashion.